The present invention relates to a circular knitting machine, particularly of the type for manufacturing socks and stockings, with a casting-off sinker actuation device.
As known, some types of knitting machines, in particular circular single-cylinder machines for manufacturing socks and stockings, have casting-off sinkers which are arranged in radial grooves of a plate-like support which is rigidly associated with the needle cylinder proximate to its upper end.
The casting-off sinkers are offset with respect to the needles, so that each sinker is arranged between two adjacent needles, and are actuated, during the rotation of the needle cylinder about its own axis, with a reciprocating motion along a direction which is radial with respect to the needle cylinder.
More particularly, the sinkers are moved away from the axis of the needle cylinder when the needles, after taking hold of the thread, begin their descent so as to facilitate the forming of new loops which rest on the upper portion of the sinkers, which is usually planar and is termed "casting-off plane", whereas the previously formed loops are cast off. After casting off the old loops, while the needles begin a new ascent, the sinkers are moved toward the axis of the needle cylinder so as to engage the new loops with a beak which is arranged above the casting-off plane, so as to tense and retain the loops against the stem of the needles.
The movement of the casting-off sinkers in a radial direction with respect to the needle cylinder is obtained by means of an inner annular cam and at least one outer countercam which are associated with a sinker ring which is arranged above the plate-like support. The annular cam and the countercam extend around the needle cylinder and define a path with portions which move toward the needle cylinder axis and portions which move away therefrom; this path is followed by a sinker heel, which protrudes upward from the plate-like support, when the needle cylinder is actuated with a rotary motion about its own axis.
In sock and stocking manufacture, the need to be able to vary the stroke of the sinkers according to the density of the knitting and to the elasticity of the thread is particularly felt.
In loose knitting, i.e. knitting with long loops, and/or with highly elastic threads, it is in fact convenient for the sinkers to be able to approach more closely the needle cylinder axis, in order to adequately tense the loops being formed, with respect to tight knitting, i.e. knitting with short loops, and/or with scarcely elastic threads for which an excessive advancement of the sinkers toward the needle cylinder axis would cause damage and breakage of the loops.
In order to satisfy this requirement, in some types of machines the countercam or countercams can yield outward in contrast with return springs. In this manner, with highly elastic threads and/or with long loops, the springs keep the countercam in the position which is closest to the inner cam, achieving a closer approach of the sinkers to the needle cylinder axis, whereas with short loops and/or with scarcely elastic threads the resistance of the knitting causes the yielding of the springs and therefore a partial outward movement of the countercam, thus avoiding an excessive approach of the sinkers to the needle cylinder axis.
However, the use of springs in order to limit the tensing of the loops has some disadvantages.
For some types of thread in some kinds of knitting, the springs employed can in fact be excessively rigid or excessively yielding with respect to the results to be obtained.
Furthermore, in machines which can reach high needle cylinder rotation rates, the centrifugal force which acts on the sinkers causes the yielding of the springs and therefore an outward spacing of the countercam or countercams, with the consequence of significantly reducing or completely eliminating the return of the sinkers toward the needle cylinder axis.
On the other hand, the use of more rigid springs can cause damage to the knitting being formed when the needle cylinder operates at low rotation rates.